Data processing systems generally comprise a central processing unit (CPU) linked to a storage subsystem through an appropriate interface and control system. High performance subsystems include one or more large capacity direct access storage devices (DASDs) such as the IBM 3380 14" disk drive for mainframe computers. High performance subsystems offer the advantages of reliability, high data capacity, and performance. For instance, DASD subsystems can operate for long periods of time before experiencing a device failure. In the event of a device failure, error recovery mechanisms such as disk mirroring assure continuous data availability.
A primary disadvantage of large form factor subsystems is the high cost of DASD. Consequently, an economical alternative has emerged comprising arrays of parallel small form factor disk drives, e.g. the 21/2" and 31/2" form factor magnetic disk drives commonly used in personal computers and laptops, or the smaller 1.8" ff magnetic disk drives. Small form factor optical disk drives are yet another alternative. Small form factor disk drive arrays emulate large form factor devices, offering advantages such as low cost, small size, and reduced power consumption. Some performance improvements may also be seen due to a large number of actuators performing parallel data accesses. However, the reliability of such arrays is poor because device failures occur more frequently than in large device subsystems.